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We characterize the co-evolution of radio-loud active galactic nuclei (AGN) and their galaxies by mapping the dependence of radio-loud AGN activity on stellar mass and star formation rate (SFR) across cosmic time (out to |$z \sim 1.5$|⁠). Deep LOFAR radio observations are combined with large galaxy samples to study the incidence of radio-loud AGN across the galaxy population; the AGN are further split into low-excitation radio galaxies (LERGs) and high-excitation radio galaxies (HERGs). We find that LERG activity occurs over a wide range of SFRs, whereas HERGs are typically found in galaxies with ongoing star formation. The LERGs are then split based on their SFRs relative to the main sequence, across redshift. Within quiescent galaxies, LERG activity shows a steep stellar mass dependence with the same normalization across the past |$\sim$| 10 Gyr; this indicates that hot gas fuels LERGs in quiescent galaxies across cosmic time. In massive galaxies [ |$\log _{10}(M/\rm {{\rm M}_{\odot }}) \gtrsim 11$| ], the incidence of LERGs is roughly constant across the galaxy population, suggesting that LERGs in massive galaxies may be fuelled by hot gas regardless of the star formation activity. At lower masses, however, LERG activity is significantly more enhanced (by a factor of up to 10) in star-forming galaxies compared to quiescent galaxies; this suggests that an additional fuelling mechanism, likely associated with cold gas, may fuel the LERGs in galaxies with higher SFRs. We find that HERGs typically accrete above 1 per cent of the Eddington-scaled accretion rate, and the LERGs typically accrete below this level. [ABSTRACT FROM AUTHOR]

Modern high-sensitivity radio telescopes are discovering an increased number of resolved sources with intricate radio structures and fainter radio emissions. These sources often present a challenge because source detectors might identify them as separate radio sources rather than components belonging to the same physically connected radio source. Currently, there are no reliable automatic methods to determine which radio components are single radio sources or part of multicomponent sources. We propose a deep-learning classifier to identify those sources that are part of a multicomponent system and require component association on data from the LOFAR Two-Metre Sky Survey. We combine different types of input data using multimodal deep learning to extract spatial and local information about the radio source components: a convolutional neural network component that processes radio images is combined with a neural network component that uses parameters measured from the radio sources and their nearest neighbours. Our model retrieves 94 per cent of the sources with multiple components on a balanced test set with 2683 sources and achieves almost 97 per cent accuracy in the real imbalanced data (323 103 sources). The approach holds potential for integration into pipelines for automatic radio component association and cross-identification. Our work demonstrates how deep learning can be used to integrate different types of data and create an effective solution for managing modern radio surveys. [ABSTRACT FROM AUTHOR]

We present rest-frame optical JWST Near-infrared Spectrograph (NIRSpec) integral field unit (IFU) spectroscopy of TN J1338−1942 at z  = 4.1, one of the most luminous galaxies in the early universe with powerful extended radio jets. Previous observations showed evidence for strong, large-scale outflows based on its large (∼150 kpc) halo detected in Ly α, and high-velocity [O  ii ] emission features detected in ground-based spectroscopy. Our NIRSpec/IFU observations spatially resolve the emission line properties across the host galaxy. We find at least five concentrations of strong line emission, coinciding with discrete continuum features previously detected in imaging from Hubble Space Telescope and JWST , over an extent of ∼2 arcsec (∼15 kpc). The spectral diagnostics unambiguously trace active galactic nuclei (AGN) activity plus interaction between the interstellar medium and the radio jet as the dominant mechanisms for the ionization state and kinematics of the gas in the system. A secondary region of very high ionization lies at roughly 5 kpc away from the nucleus, and within the context of an expanding cocoon enveloping the radio lobe, this may be explained by strong shock-ionization of the entrained gas. However, it could also signal the presence of a second obscured AGN, which may offer an explanation for an intriguing outflow feature seen perpendicular to the radio axis. The presence of a dual supermassive black holes system in this galaxy would support that large galaxies in the early Universe quickly accumulated their mass through the merging of smaller units, at the centres of large overdensities. The inferred black hole mass to stellar mass ratio of 0.01−0.1 for TNJ1338 points to a more rapid assembly of black holes compared to the stellar mass of galaxies at high redshifts, consistent with other recent observations. [ABSTRACT FROM AUTHOR]

Context. Many massive galaxies launch jets from the accretion disk of their central black hole, but only 03 instances are known in which the associated outflows form giant radio galaxies (GRGs, or giants): luminous structures of megaparsec extent that consist of atomic nuclei, relativistic electrons, and magnetic fields. Large samples are imperative to understanding the enigmatic growth of giants, and recent systematic searches in homogeneous surveys constitute a promising development. For the first time, it is possible to perform meaningful precision statistics with GRG lengths, but a framework to do so is missing. Aims. We measured the intrinsic GRG length distribution by combining a novel statistical framework with a LOFAR Two-metre Sky Survey (LoTSS) sample of freshly discovered giants. In turn, this allowed us to answer an array of questions on giants. For example, we can now assess how rare a 5 Mpc giant is compared with one of 1 Mpc, and how much larger - given a projected length - the corresponding intrinsic length is expected to be. Notably, we can now also infer the GRG number density in the Local Universe. Methods. We assumed the intrinsic GRG length distribution to be Paretian (i.e. of power-law form) with tail index, and predicted the observed distribution by modelling projection and selection effects. To infer, we also systematically searched the LoTSS for hitherto unknown giants and compiled the largest catalogue of giants to date. Results. We show that if intrinsic GRG lengths are Pareto distributed with index, then projected GRG lengths are also Pareto distributed with index. Selection effects induce curvature in the observed projected GRG length distribution: angular length selection flattens it towards the lower end, while surface brightness selection steepens it towards the higher end. We explicitly derived a GRG's posterior over intrinsic lengths given its projected length, laying bare the dependence. We also discovered 2060 giants within LoTSS DR2 pipeline products; our sample more than doubles the known population. Spectacular discoveries include the largest, second-largest, and fourth-largest GRG known (lp = 5:1 Mpc, lp = 5:0 Mpc, and lp = 4:8 Mpc), the largest GRG known hosted by a spiral galaxy (lp = 2:5 Mpc), and the largest secure GRG known beyond redshift 1 (lp = 3:9 Mpc). We increase the number of known giants whose angular length exceeds that of the Moon from 10 to 23; among the discoveries is the angularly largest known radio galaxy in the Northern Sky, which is also the angularly largest known GRG (= 2). Combining theory and data, we determined that intrinsic GRG lengths are well described by a Pareto distribution, and measured the index = 3:5:5. This implies that, given its projected length, a GRG's intrinsic length is expected to be just 15% larger. Finally, we determined the comoving number density of giants in the Local Universe to be nGRG = 5 100 Mpc 3. Conclusions. We developed a practical mathematical framework that elucidates the statistics of giant radio galaxy lengths. Through a LoTSS search, we also discovered 2060 new giants. By combining both advances, we determined that intrinsic GRG lengths are well described by a Pareto distribution with index = 3:5 = 0:5, and that giants are truly rare in a cosmological sense: most clusters and filaments of the CosmicWeb are not currently home to a giant. Thus, our work yields new observational constraints for analytical models and simulations featuring radio galaxy growth. [ABSTRACT FROM AUTHOR]

The warm–hot intergalactic medium (warm–hot IGM, or WHIM) pervades the filaments of the Cosmic Web and harbours half of the Universe's baryons. The WHIM's thermodynamic properties are notoriously hard to measure. Here we estimate a galaxy group–WHIM boundary temperature using a new method. In particular, we use a radio image of the giant radio galaxy (giant RG, or GRG) created by NGC 6185, a massive nearby spiral. We analyse this extraordinary object with a Bayesian 3D lobe model and deduce an equipartition pressure P eq = 6 × 10−16 Pa – among the lowest found in RGs yet. Using an X-ray-based statistical conversion for Fanaroff–Riley II RGs, we find a true lobe pressure |$P = 1.5\substack{+1.7\\ -0.4}\times 10^{-15}\ \mathrm{Pa}$|⁠. Cosmic Web reconstructions, group catalogues, and MHD simulations furthermore imply an Mpc–scale IGM density |$1 + \delta _\mathrm{IGM} = 40\substack{+30\\ -10}$|⁠. The buoyantly rising lobes are crushed by the IGM at their inner side, where an approximate balance between IGM and lobe pressure occurs: P IGM ≈ P. The ideal gas law then suggests an IGM temperature |$T_\mathrm{IGM} = 11\substack{+12\\-5} \times 10^6\ \mathrm{K}$|⁠ , or |$k_\mathrm{B}T_\mathrm{IGM} = 0.9\substack{+1.0\\-0.4}\ \mathrm{keV}$|⁠ , at the virial radius – consistent with X-ray-derived temperatures of similarly massive groups. Interestingly, the method is not performing at its limit: in principle, estimates T IGM ∼ 4 × 106 K are already possible – rivalling the lowest X-ray measurements available. The technique's future scope extends from galaxy group outskirts to the WHIM. In conclusion, we demonstrate that observations of GRGs in Cosmic Web filaments are finally sensitive enough to probe the thermodynamics of galaxy groups and beyond. [ABSTRACT FROM AUTHOR]

Feedback from low-excitation radio galaxies (LERGs) plays a key role in the lifecycle of massive galaxies in the local Universe; their evolution, and the impact of these active galactic nuclei on early galaxy evolution, however, remain poorly understood. We use a sample of 10 481 LERGs from the first data release of the LOFAR two-metre Sky Survey Deep Fields, covering ∼25 deg2, to present the first measurement of the evolution of the radio luminosity function (LF) of LERGs out to z ∼ 2.5; this shows relatively mild evolution. We split the LERGs into those hosted by quiescent and star-forming galaxies, finding a new dominant population of LERGs hosted by star-forming galaxies at high redshifts. The incidence of LERGs in quiescent galaxies shows a steep dependence on stellar mass out to z ∼ 1.5, consistent with local Universe measurements of accretion occurring from cooling of hot gas haloes. The quiescent-LERGs dominate the LFs at z < 1, showing a strong decline in space density with redshift, tracing that of the available host galaxies, while there is an increase in the characteristic luminosity. The star-forming LERG LF increases with redshift, such that this population dominates the space densities at most radio-luminosities by z ∼ 1. The incidence of LERGs in star-forming galaxies shows a much weaker stellar-mass dependence, and increases with redshift, suggesting a different fuelling mechanism compared to their quiescent counterparts, potentially associated with the cold gas supply present in the star-forming galaxies. [ABSTRACT FROM AUTHOR]

Building on the first paper in this series (Duncan et al. 2018), we present a study investigating the performance of Gaussian process photometric redshift (photo-z) estimates for galaxies and active galactic nuclei (AGNs) detected in deep radio continuum surveys. A Gaussian process redshift code is used to produce photo-z estimates targeting specific subsets of both the AGN population -- infrared (IR), X-ray, and optically selected AGNs -- and the general galaxy population. The new estimates for the AGN population are found to perform significantly better at z > 1 than the template-based photo-z estimates presented in our previous study. Our new photo-z estimates are then combined with template estimates through hierarchical Bayesian combination to produce a hybrid consensus estimate that outperforms both of the individual methods across all source types. Photo-z estimates for radio sources that are X-ray sources or optical/IR AGNs are significantly improved in comparison to previous template- only estimates -- with outlier fractions and robust scatter reduced by up to a factor of -4. The ability of our method to combine the strengths of the two input photo-z techniques and the large improvements we observe illustrate its potential for enabling future exploitation of deep radio continuum surveys for both the study of galaxy and black hole coevolution and for cosmological studies. [ABSTRACT FROM AUTHOR]

We present a study of photometric redshift performance for galaxies and active galactic nuclei detected in deep radio continuum surveys. Using two multiwavelength data sets, over the NOAO Deep Wide Field Survey Boötes and COSMOS fields, we assess photometric redshift (photo-z) performance for a sample of ∼4500 radio continuum sources with spectroscopic redshifts relative to those of ∼63 000 non-radio-detected sources in the same fields. We investigate the performance of three photometric redshift template sets as a function of redshift, radio luminosity and infrared/X-ray properties.We find that no single template library is able to provide the best performance across all subsets of the radio-detected population, with variation in the optimum template set both between subsets and between fields. Through a hierarchical Bayesian combination of the photo-z estimates from all three template sets, we are able to produce a consensus photo-z estimate that equals or improves upon the performance of any individual template set. [ABSTRACT FROM AUTHOR]

We present a sample of ∼1000 emission-line galaxies atz = 0.4-4.7 from the ∼0.7deg2 High-z Emission-Line Survey in the Boötes field identified with a suite of six narrow-band filters at ≈0.4-2.1 µm. These galaxies have been selected on their Ly α (73), [O II] (285), H β/[O III] (387) or H α (362) emission line, and have been classified with optical to near-infrared colours. A subsample of 98 sources have reliable redshifts from multiple narrow-band (e.g. [O II]-H α) detections and/or spectroscopy. In this survey paper, we present the observations, selection and catalogues of emitters. We measure number densities of Ly α, [O II], H β/[O III] and H α and confirm strong luminosity evolution in star-forming galaxies from z ∼ 0.4 to ∼5, in agreement with previous results. To demonstrate the usefulness of dual-line emitters, we use the sample of dual [O II]-H α emitters to measure the observed [O II]/H α ratio at z = 1.47. The observed [O II]/H α ratio increases significantly from 0.40 ± 0.01 at z = 0.1 to 0.52 ± 0.05 at z = 1.47, which we attribute to either decreasing dust attenuation with redshift, or due to a bias in the (typically) fibre measurements in the local Universe that only measure the central kpc regions. At the bright end, we find that both the H α and Ly α number densities at z ≈ 2.2 deviate significantly from a Schechter form, following a power law. We show that this is driven entirely by an increasing X-ray/active galactic nucleus fraction with line luminosity, which reaches ≈100 per cent at line luminosities L 3 × 1044 erg s-1. [ABSTRACT FROM AUTHOR]

In this paper, the second in a series investigating Fanaroff–Riley type II (FR II) radio galaxies at low frequencies, we use LOw Frequency ARray (LOFAR) and Very Large Array (VLA) observations between 117 and 456 MHz, in addition to archival data, to determine the dynamics and energetics of two radio galaxies, 3C 452 and 3C 223, by fitting spectral ageing models on small spatial scales. We provide improved measurements for the physical extent of the two sources, including a previously unknown low surface brightness extension to the northern lobe of 3C 223, and revised energetics based on these values. We find spectral ages of 77.05+9.22 −8.74 and 84.96+15.02 −13.83 Myr for 3C 452 and 3C 223, respectively, suggesting a characteristic advance speed for the lobes of around 1 per cent of the speed of light. For 3C 452, we show that, even for a magnetic field strength not assumed to be in equipartition, a disparity of a factor of approximately 2 exists between the spectral age and that determined from a dynamical standpoint. We confirm that the injection index of both sources (as derived from the lobe emission) remains steeper than classically assumed values, even when considered on well-resolved scales at low frequencies. However, we find an unexpected sharp discontinuity between the spectrum of the hotspots and the surrounding lobe emission. We suggest that this discrepancy is a result of the absorption of hotspot emission and/or non-homogeneous and additional acceleration mechanisms; as such, hotspots should not be used in the determination of the underlying initial electron energy distribution. [ABSTRACT FROM AUTHOR]

The correlation between radio spectral index and redshift has been exploited to discover highredshift radio galaxies, but its underlying cause is unclear. It is crucial to characterize the particle acceleration and loss mechanisms in high-redshift radio galaxies to understand why their radio spectral indices are steeper than their local counterparts. Low-frequency information on scales of ~1 arcsec are necessary to determine the internal spectral index variation. In this paper we present the first spatially resolved studies at frequencies below 100MHz of the z = 2.4 radio galaxy 4C 43.15 which was selected based on its ultrasteep spectral index (α < -1; Sv ~ vα) between 365MHz and 1.4 GHz. Using the International Low Frequency Array Low Band Antenna we achieve subarcsecond imaging resolution at 55MHz with very long baseline interferometry techniques. Our study reveals low-frequency radio emission extended along the jet axis, which connects the two lobes. The integrated spectral index for frequencies <500MHz is -0.83. The lobes have integrated spectral indices of -1.31 ± 0.03 and -1.75 ± 0.01 for frequencies ≥1.4 GHz, implying a break frequency between 500MHz and 1.4 GHz. These spectral properties are similar to those of local radio galaxies. We conclude that the initially measured ultrasteep spectral index is due to a combination of the steepening spectrum at high frequencies with a break at intermediate frequencies. [ABSTRACT FROM AUTHOR]

Due to their steep spectra, low-frequency observations of Fanaroff-Riley type II (FR II) radio galaxies potentially provide key insights in to the morphology, energetics and spectrum of these powerful radio sources. However, limitations imposed by the previous generation of radio interferometers at metre wavelengths have meant that this region of parameter space remains largely unexplored. In this paper, the first in a series examining FR IIs at low frequencies, we use LOFAR (LOw Frequency ARray) observations between 50 and 160 MHz, along with complementary archival radio and X-ray data, to explore the properties of two FR II sources, 3C 452 and 3C 223. We find that the morphology of 3C 452 is that of a standard FR II rather than of a double-double radio galaxy as had previously been suggested, with no remnant emission being observed beyond the active lobes. We find that the low-frequency integrated spectra of both sources are much steeper than expected based on traditional assumptions and, using synchrotron/inverse-Compton model fitting, show that the total energy content of the lobes is greater than previous estimates by a factor of around 5 for 3C 452 and 2 for 3C 223. We go on to discuss possible causes of these steeper-than-expected spectra and provide revised estimates of the internal pressures and magnetic field strengths for the intrinsically steep case. We find that the ratio between the equipartition magnetic field strengths and those derived through synchrotron/inverse-Compton model fitting remains consistent with previous findings and show that the observed departure from equipartition may in some cases provide a solution to the spectral versus dynamical age disparity. [ABSTRACT FROM AUTHOR]

CIZA J2242.8+5301 (z = 0.188, nicknamed 'Sausage') is an extremely massive (M200 ~ 2.0 × 1015 M⊙), merging cluster with shock waves towards its outskirts, which was found to host numerous emission line galaxies. We performed extremely deep Westerbork Synthesis Radio Telescope H I observations of the 'Sausage' cluster to investigate the effect of the merger and the shocks on the gas reservoirs fuelling present and future star formation (SF) in cluster members. By using spectral stacking, we find that the emission line galaxies in the 'Sausage' cluster have, on average, as much H I gas as field galaxies (when accounting for the fact cluster galaxies are more massive than the field galaxies), contrary to previous studies. Since the cluster galaxies are more massive than the field spirals, they may have been able to retain their gas during the cluster merger. The large H I reservoirs are expected to be consumed within ~0.75-1.0 Gyr by the vigorous SF and active galactic nuclei activity and/or driven out by the outflows we observe. We find that the star formation rate (SFR) in a large fraction of H α emission line cluster galaxies correlates well with the radio broad-band emission, tracing supernova remnant emission. This suggests that the cluster galaxies, all located in post-shock regions, may have been undergoing sustained SFR for at least 100 Myr. This fully supports the interpretation proposed by Stroe et al. and Sobral et al. that gas-rich cluster galaxies have been triggered to form stars by the passage of the shock. [ABSTRACT FROM AUTHOR]

At very low frequencies, the new pan-European radio telescope, LOFAR, is opening the last unexplored window of the electromagnetic spectrum for astrophysical studies. Operating at frequencies from 15 to 240 MHz, its superb sensitivity, high angular resolution, large field of view and flexible spectroscopic capabilities represent a dramatic improvement over previous facilities at these wavelengths. LOFAR will carry out a broad range of fundamental astrophysical studies in a number of key science topics including the formation and evolution of clusters, galaxies and black holes. In this contribution we describe some of the capabilities of LOFAR and present some recent results from the ongoing imaging efforts. We also discuss the impact of LOFAR on our studies of radio-loud AGN. Our recent study of the evolution of radio-loud AGN as a function of host stellar mass shows a clear increase in the fraction of lower mass galaxies which host radio-loud AGN at 1 < z < 2 while the fraction for higher mass galaxies remains the same. This shows that the upturn in the radio luminosity function is driven by increasing AGN activity among low mass galaxies at higher redshifts. New LOFAR observations will allow us to build statistically large samples at high redshifts to constrain this evolution for the different accretion modes of AGN. [ABSTRACT FROM PUBLISHER]

At very low frequencies, the new pan-European radio telescope, LOFAR, is opening the last unexplored window of the electromagnetic spectrum for astrophysical studies. LOFAR will deeply survey the northern sky from ~ 10 up to 240 MHz. In this contribution we briefly describe some of the capabilities of LOFAR and the surveys planned to study fundamental issues related to the formation and evolution of galaxies and clusters of galaxies. We describe some of the challenges of low frequency observations with LOFAR and our progress in overcoming them. Further, we present some recent results from the ongoing imaging commissioning efforts. In the second part we discuss our studies of Low Excitation and High Excitation Radio Galaxies in the Boötes field and how LOFAR Surveys will help in studying their evolution. [ABSTRACT FROM PUBLISHER]

In this paper we present near-infraredK-band imaging of a sample of ultra-steep-spectrum (USS) radio sources selected at 74 MHz. The dual selection criteria of low frequency and USS mean that we should be sensitive to the highest-redshift (z>5) radio galaxies. We have obtainedK-band magnitudes for all of the objects in our sample of 26 and discuss the properties of each.There is a pronounced bias in this sample towards fainter magnitudes and thus higher redshifts when compared to complete unfiltered samples such as the 7CRS of Willott et al., implying that the steep-spectrum technique is still viable at 74 MHz. However, there are more bright (K<17) sources in the 74-MHz sample than in a similar sample selected at 151 MHz, namely 6C*. This is principally due to the additional selection criterion of a small angular size for the radio sources in 6C*; four of the six sources in the 74-MHz USS sample withK<17 have angular sizes>15 arcsec (the angular size cut-off of 6C*).We find that the distribution ofK-band magnitudes from a sample selected at 74 MHz is statistically indistinguishable from the 6C* sample, when similar angular size filtering is applied to the 74-MHz sample. [ABSTRACT FROM AUTHOR]

We present the results of an observing programme with the Submillimetre Common User Bolometer Array (SCUBA) to measure the submillimetre (submm) dust continuum emission of 24 distant radio galaxies. We detected submm emission in 12 galaxies with signal-to-noise > 3, including nine detections at . When added to previous published results these data almost triple the number of radio galaxies with detected in the submm and yield a sample of 69 observed radio galaxies over the redshift range . We find that the range in rest-frame far-infrared luminosities is about a factor of 10. We have investigated the origin of this dispersion, correlating the luminosities with radio source power, size, spectral index, K-band magnitude and Lyα luminosity. No strong correlations are apparent in the combined data set. We confirm and strengthen the result from previous submm observations of radio galaxies that the detection rate is a strong function of redshift. We compare the redshift dependence of the submm properties of radio galaxies with those of quasars and find that for both classes of objects the observed submm flux density increases with redshift to , beyond which, for the galaxies, we find tentative evidence for a decline. We find evidence for an anticorrelation between submm luminosity and ultraviolet polarization fraction, for a subsample of 13 radio galaxies, indicating that starbursts are the dominant source of heating for dust in radio galaxies. [ABSTRACT FROM AUTHOR]

The powerful radio galaxy PKS 1138−262 is a massive galaxy in a dense environment as indicated by optical, NIR, radio and X-ray observations. We have found an overdensity of Lyα emitters within 1.5 Mpc of the radio galaxy which form a proto cluster at z=2.2. We have also found numerous candidate Hα emitters near 1138−262. The surface density of Hα candidates increases towards the radio galaxy, contrary to the density of Lyα candidates. We have spectroscopically confirmed the identification of nine Hα candidates whose velocity dispersion is smaller than for the Lyα emitting sample. The field of 1138−262 also contains a high density of EROs and X-ray emitters. We will discuss the implications of these measurements and show that there are several galaxy populations present in the proto cluster. [Copyright &y& Elsevier]

The clustering properties of objects in three different radio surveys (NVSS, FIRST and BOOTES-WSRT) and two near-infrared surveys (the ‘Daddi field’ and the FIRES survey) are investigated and compared with studies of various samples of galaxies, AGN and clusters. At z∼1, it seems that the 2dF optical quasars have a correlation length a factor of about 2 less than powerful radio galaxies at similar redshifts. This indicates that these two classes of object cannot be ‘unified’ by postulating that their main difference is due to their evolution being at a different stage. It seems much more likely that these QSOs are predominantly located in field galaxies, while the powerful radio sources are located preferentially in early types. Furthermore, it appears that both the extremely red objects (EROs) from the IR surveys and the more luminous radio sources are similarly clustered and as such are the most clustered objects known in the z>1 Universe. Even at z∼3 the red J−K galaxies from the FIRES survey are similarly strongly clustered, at a level of about three times higher than Lyman break galaxies. These clustering properties are consistent with EROs and radio galaxies being similar objects at different stages of their evolution. Locally, the most clustered population of objects are clusters of galaxies. Since the progenitors of these objects—proto-clusters—will therefore also be highly clustered, a good way to locate proto-clusters is to target fields with very powerful and potentially highly clustered distant z>2 radio sources. The techniques that are currently being used for locating and studying these proto-clusters are briefly discussed. [Copyright &y& Elsevier]

Narrowband Ly α imaging has been carried out around eight radio galaxies at 2 to search for (proto-)cluster galaxies at the same redshift. Subsequent spectroscopy has led to the discovery of six protoclusters at redshifts z=2.2, 2.9, 2.9, 3.1, 3.2 and 4.1, with velocity dispersions between 300 km s−1 and 1000 km s−1. The estimated masses are in the range 1014–1016 M⊙, comparable to the mass of present day rich clusters of galaxies. [Copyright &y& Elsevier]